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.TH CIRCLEQ 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
CIRCLEQ_EMPTY,
CIRCLEQ_ENTRY,
CIRCLEQ_FIRST,
CIRCLEQ_FOREACH,
CIRCLEQ_FOREACH_REVERSE,
CIRCLEQ_HEAD,
CIRCLEQ_HEAD_INITIALIZER,
CIRCLEQ_INIT,
CIRCLEQ_INSERT_AFTER,
CIRCLEQ_INSERT_BEFORE,
CIRCLEQ_INSERT_HEAD,
CIRCLEQ_INSERT_TAIL,
CIRCLEQ_LAST,
CIRCLEQ_LOOP_NEXT,
CIRCLEQ_LOOP_PREV,
CIRCLEQ_NEXT,
CIRCLEQ_PREV,
CIRCLEQ_REMOVE
\- implementation of a doubly linked circular queue
.SH SYNOPSIS
.nf
.B #include <sys/queue.h>
.PP
.B CIRCLEQ_ENTRY(TYPE);
.PP
.B CIRCLEQ_HEAD(HEADNAME, TYPE);
.BI "CIRCLEQ_HEAD CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD " head );
.BI "void CIRCLEQ_INIT(CIRCLEQ_HEAD *" head );
.PP
.BI "int CIRCLEQ_EMPTY(CIRCLEQ_HEAD *" head );
.PP
.BI "void CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *" head ,
.BI "                           struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "void CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *" head ,
.BI "                           struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "void CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *" head ", struct TYPE *" listelm ,
.BI "                           struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "void CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *" head ", struct TYPE *" listelm ,
.BI "                           struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.PP
.BI "struct TYPE *CIRCLEQ_FIRST(CIRCLEQ_HEAD *" head );
.BI "struct TYPE *CIRCLEQ_LAST(CIRCLEQ_HEAD *" head );
.BI "struct TYPE *CIRCLEQ_PREV(struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "struct TYPE *CIRCLEQ_NEXT(struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "struct TYPE *CIRCLEQ_LOOP_PREV(CIRCLEQ_HEAD *" head ,
.BI "                           struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "struct TYPE *CIRCLEQ_LOOP_NEXT(CIRCLEQ_HEAD *" head ,
.BI "                           struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.PP
.BI "CIRCLEQ_FOREACH(struct TYPE *" var ", CIRCLEQ_HEAD *" head ,
.BI "                           CIRCLEQ_ENTRY " NAME );
.BI "CIRCLEQ_FOREACH_REVERSE(struct TYPE *" var ", CIRCLEQ_HEAD *" head ,
.BI "                           CIRCLEQ_ENTRY " NAME );
.PP
.BI "void CIRCLEQ_REMOVE(CIRCLEQ_HEAD *" head ", struct TYPE *" elm ,
.BI "                           CIRCLEQ_ENTRY " NAME );
.fi
.SH DESCRIPTION
These macros define and operate on doubly linked circular queues.
.PP
In the macro definitions,
.I TYPE
is the name of a user-defined structure,
that must contain a field of type
.IR CIRCLEQ_ENTRY ,
named
.IR NAME .
The argument
.I HEADNAME
is the name of a user-defined structure
that must be declared using the macro
.BR CIRCLEQ_HEAD ().
.SS Creation
A circular queue is headed by a structure defined by the
.BR CIRCLEQ_HEAD ()
macro.
This structure contains a pair of pointers,
one to the first element in the queue
and the other to the last element in the queue.
The elements are doubly linked
so that an arbitrary element can be removed without traversing the queue.
New elements can be added to the queue
after an existing element,
before an existing element,
at the head of the queue,
or at the end of the queue.
A
.I CIRCLEQ_HEAD
structure is declared as follows:
.PP
.in +4
.EX
CIRCLEQ_HEAD(HEADNAME, TYPE) head;
.EE
.in
.PP
where
.I struct HEADNAME
is the structure to be defined, and
.I struct TYPE
is the type of the elements to be linked into the queue.
A pointer to the head of the queue can later be declared as:
.PP
.in +4
.EX
struct HEADNAME *headp;
.EE
.in
.PP
(The names
.I head
and
.I headp
are user selectable.)
.PP
.BR CIRCLEQ_ENTRY ()
declares a structure that connects the elements in the queue.
.PP
.BR CIRCLEQ_HEAD_INITIALIZER ()
evaluates to an initializer for the queue
.IR head .
.PP
.BR CIRCLEQ_INIT ()
initializes the queue referenced by
.IR head .
.PP
.BR CIRCLEQ_EMPTY ()
evaluates to true if there are no items on the queue.
.SS Insertion
.BR CIRCLEQ_INSERT_HEAD ()
inserts the new element
.I elm
at the head of the queue.
.PP
.BR CIRCLEQ_INSERT_TAIL ()
inserts the new element
.I elm
at the end of the queue.
.PP
.BR CIRCLEQ_INSERT_BEFORE ()
inserts the new element
.I elm
before the element
.IR listelm .
.PP
.BR CIRCLEQ_INSERT_AFTER ()
inserts the new element
.I elm
after the element
.IR listelm .
.SS Traversal
.BR CIRCLEQ_FIRST ()
returns the first item on the queue.
.PP
.BR CIRCLEQ_LAST ()
returns the last item on the queue.
.PP
.BR CIRCLEQ_PREV ()
returns the previous item on the queue, or
.I &head
if this item is the first one.
.PP
.BR CIRCLEQ_NEXT ()
returns the next item on the queue, or
.I &head
if this item is the last one.
.PP
.BR CIRCLEQ_LOOP_PREV ()
returns the previous item on the queue.
If
.I elm
is the first element on the queue, the last element is returned.
.PP
.BR CIRCLEQ_LOOP_NEXT ()
returns the next item on the queue.
If
.I elm
is the last element on the queue, the first element is returned.
.PP
.BR CIRCLEQ_FOREACH ()
traverses the queue referenced by
.I head
in the forward direction, assigning each element in turn to
.IR var .
.I var
is set to
.I &head
if the loop completes normally, or if there were no elements.
.PP
.BR CIRCLEQ_FOREACH_REVERSE ()
traverses the queue referenced by
.I head
in the reverse direction,
assigning each element in turn to
.IR var .
.SS Removal
.BR CIRCLEQ_REMOVE ()
removes the element
.I elm
from the queue.
.SH RETURN VALUE
.BR CIRCLEQ_EMPTY ()
returns nonzero if the queue is empty,
and zero if the queue contains at least one entry.
.PP
.BR CIRCLEQ_FIRST (),
.BR CIRCLEQ_LAST (),
.BR CIRCLEQ_LOOP_PREV (),
and
.BR CIRCLEQ_LOOP_NEXT ()
return a pointer to the first, last, previous, or next
.I TYPE
structure, respectively.
.PP
.BR CIRCLEQ_PREV (),
and
.BR CIRCLEQ_NEXT ()
are similar to their
.BR CIRCLEQ_LOOP_* ()
counterparts,
except that if the argument is the first or last element, respectively,
they return
.IR &head .
.PP
.BR CIRCLEQ_HEAD_INITIALIZER ()
returns an initializer that can be assigned to the queue
.IR head .
.SH CONFORMING TO
Not in POSIX.1, POSIX.1-2001, or POSIX.1-2008.
Present on the BSDs
(CIRCLEQ macros first appeared in 4.4BSD).
.SH BUGS
.BR CIRCLEQ_FOREACH ()
and
.BR CIRCLEQ_FOREACH_REVERSE ()
don't allow
.I var
to be removed or freed within the loop,
as it would interfere with the traversal.
.BR CIRCLEQ_FOREACH_SAFE ()
and
.BR CIRCLEQ_FOREACH_REVERSE_SAFE (),
which are present on the BSDs but are not present in glibc,
fix this limitation by allowing
.I var
to safely be removed from the list and freed from within the loop
without interfering with the traversal.
.SH EXAMPLES
.EX
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>

struct entry {
    int data;
    CIRCLEQ_ENTRY(entry) entries;           /* Queue */
};

CIRCLEQ_HEAD(circlehead, entry);

int
main(void)
{
    struct entry *n1, *n2, *n3, *np;
    struct circlehead head;                 /* Queue head */
    int i;

    CIRCLEQ_INIT(&head);                    /* Initialize the queue */

    n1 = malloc(sizeof(struct entry));      /* Insert at the head */
    CIRCLEQ_INSERT_HEAD(&head, n1, entries);

    n1 = malloc(sizeof(struct entry));      /* Insert at the tail */
    CIRCLEQ_INSERT_TAIL(&head, n1, entries);

    n2 = malloc(sizeof(struct entry));      /* Insert after */
    CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);

    n3 = malloc(sizeof(struct entry));      /* Insert before */
    CIRCLEQ_INSERT_BEFORE(&head, n2, n3, entries);

    CIRCLEQ_REMOVE(&head, n2, entries);     /* Deletion */
    free(n2);
                                            /* Forward traversal */
    i = 0;
    CIRCLEQ_FOREACH(np, &head, entries)
        np\->data = i++;
                                            /* Reverse traversal */
    CIRCLEQ_FOREACH_REVERSE(np, &head, entries)
        printf("%i\en", np\->data);
                                            /* Queue deletion */
    n1 = CIRCLEQ_FIRST(&head);
    while (n1 != (void *)&head) {
        n2 = CIRCLEQ_NEXT(n1, entries);
        free(n1);
        n1 = n2;
    }
    CIRCLEQ_INIT(&head);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR insque (3),
.BR queue (7)
.SH COLOPHON
This page is part of release 5.13 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.
